1. Field of the Invention
The present invention relates to a driving force control system for four-wheel drive vehicles comprising main driven wheels which are driven by an engine via a torque converter and auxiliary driven wheels which are driven by an electric motor independently from the engine.
2. Description of the Prior Art
Hybrid type four-wheel drive vehicles in which a pair of front or rear main driven wheels are driven by an engine and another pair of front or rear auxiliary driven wheels are driven by an electric motor which is connected to a battery are known in, for example, Japanese Patent Application Laid-open No. 5-8639 and Japanese Patent Application Laid-open No. 6-225403. Such four-wheel drive vehicles attempt to economize on fuel consumption and improve the ground covering properties on poor condition roads by switching over between a two-wheel drive state in which the vehicle travels using the engine alone and a four-wheel drive state in which the vehicle travels using the engine and the electric motor on the basis of a switch-over pattern which is predetermined using the engine load and the vehicle speed as parameters.
In the case of front-wheel drive vehicles in which the engine is mounted on the front section, since the percentage torque amplification is large in the region in which the speed ratio of the torque converter is small, such as when starting to travel. The front wheels, which are the main driven wheels, easily slip on slippery road surfaces such as roads with snow. It is therefore possible to suppress in advance the slip of the front wheels by distributing the driving force to the rear wheels which are auxiliary driven wheels. Moreover, the response of the vehicle to depression of the accelerator pedal can be enhanced by applying assistance from an electric motor in the region in which the speed ratio of the torque converter, which employs the transmission of a fluid, is small. The efficiency xcex7 of the torque converter is a function of the speed ratio e (output rotational rate / input rotational rate). The efficiency xcex7 is low in the region where the speed ratio e is small such as when the vehicle is starting to travel, and the efficiency xcex7 is high in the region where the speed ratio e is large such as when the vehicle is cruising. Therefore, if the engine is assisted by accurately operating the electric motor in the region where the speed ratio e of the torque converter is small such as when the vehicle is starting, it can contribute to economizing the fuel consumption of the engine.
The present invention has been made in view of the above-mentioned circumstances and, with regard to a four-wheel drive vehicle whose main driven wheels are driven by an engine via a torque converter and whose auxiliary driven wheels are driven by an electric motor. It is an objective of the present invention to improve the response of the vehicle to the operation of the accelerator by suppressing the slip of the main driven wheels when starting to travel, etc.
In order to achieve this object, a driving force control system for a four-wheel drive vehicle is provided which comprises main driven wheels driven by an engine via a torque converter, speed ratio detecting means for detecting the speed ratio of the torque converter and auxiliary driven wheels driven by an electric motor independently from the engine. A control means controls the distribution ratio of the output generated by the engine and the output generated by the electric motor according to the speed ratio of the torque converter.
Since the output generated by the engine and the output generated by the electric motor are distributed according to the speed ratio of the torque converter, by reducing the output of the engine for driving the main driven wheels and increasing the output of the electric motor for driving the auxiliary driven wheels in the region in which the speed ratio of the torque converter is low and the percentage torque amplification is high when the vehicle is starting, the driving force can be distributed among the four wheels so as to suppress the slip of the main driven wheels and improve the response of the vehicle to the operation of the accelerator. By assisting the output of the engine by means of the output of the electric motor so as to compensate for the reduction in the torque converter efficiency xcex7 in the region in which the speed ratio is low, it can be anticipated that the effect will be to reduce the fuel consumption.
Furthermore, a driving force control system for a four-wheel drive vehicle is proposed wherein the speed ratio of the torque converter at which the drive of the electric motor is suspended, different from the speed ratio of the torque converter at which the drive of the electric motor is started.
Since the speed ratio at which the drive of the electric motor is suspended is made different from the speed ratio at which the drive of the electric motor is started, it is possible to prevent frequent switch-over between driving and stopping or suspending the drive of the electric motor when the speed ratio changes slightly.
Still further, a driving force control system for a four-wheel drive vehicle is proposed wherein the maximum output of the electric motor changes according to the state of slip of the main driven wheels.
Since the maximum output of the electric motor is changed according to the state of slip of the main driven wheels, it is possible to effectively suppress the slip of the main driven wheels on a road surface having a low coefficient of friction when the vehicle is starting, etc. thereby enhancing the ground covering properties.